The contents inside reads:

We were deeply touched by your most kind and thoughtful message following our Coronation. We are enormously grateful to everyone who took part in the celebrations, and particularly appreciate that you so generously took the time to write to us on this very special occasion.

CHARLES R CAMILLA R

EDIT: The below paragraph is false, it’s very similar to The Queen’s cypher but has an extra C, so appears to be an impaled/joint cypher

The message is signed by both of them, however the envelope lacked the Royal Cypher and the front of the card uses The Queen’s Cypher, so I presume that it’s from her.